Method for evaluating an input data signal and circuit system for carrying out said method

ABSTRACT

A method, a circuit arrangement and an apparatus are provided for evaluating an input data signal transmitted by load modulation. A complex data signal is derived from the input data and a mean value signal of the complex data signal is thereafter derived. A complex signal without mean value is also formed from the difference between the complex data signal and the mean value signal of the complex data signal. A first and second quadratic error signals are derived and subsequently a slope signal is derived from the first and second quadratic signals. The method also comprises deriving an information signal by comparing the imaginary part signal without mean value with a decision threshold signal. The information signal indicates whether a value of the input data signal has been generated in a loaded state or an unloaded state during the load modulation.

FIELD OF THE INVENTION

The invention relates to a method for evaluating an input data signalgenerated by load modulation, and also to a circuit arrangement forcarrying out such a method.

BACKGROUND ART

In magnetically coupled, contactless identification systems consistingof one or more contactless data carriers and a write/read apparatus, thedata transmission from the data 5 carrier to the write/read apparatusand vice versa is tested on the basis of the so-called maximumread/write distance. The maximum read/write distance offers an importantcriterion as regards the operation of the devices, notably when thewrite/read apparatus and the data carrier (carriers) are to operate onlyin the tested combination.

The contactless data carriers are magnetically coupled to the write/readapparatus. The energy for the power supply of the data carrier as wellas the data signals themselves are transferred via the magneticcoupling. The transfer of the data signals from the data carrier to thewrite/read apparatus takes place via load modulation. To this end, anadditional impedance, referred to as a load impedance, is switched onand off by the data signal on the data carrier. An arrangement of thiskind is shown in general form in FIG. 1. An inductance 1 of thewrite/read apparatus and an inductance 2 of the data carrier thereinconstitute the arrangement for the magnetic coupling of the data signalsand the power supply.

Parallel to the inductance 2 of the data carrier there is connected afirst impedance 3 which serves as a fixed load, and also a loadimpedance 5 which can be activated via a switch 4. The switch 4 ispreferably electronically controlled by the data signal to be appliedfrom the data carrier to the write/read apparatus (see the arrow 6).

For independent development of the write/read apparatus on the one handand the data carrier on the other hand, and for definition of theoperating parameters, specification of these two components is necessaryin the sense of standardization according to the ISO standard 14.443. Inthis context it is to be noted that the standard ISO/IECJTC1/SC17/WG8/TF2 proposes a measuring arrangement for determining themodulation of the data carrier, by means of a coil system which isdiagrammatically shown in FIG. 2. The mechanical and electrical data ofthis measuring arrangement are defined in the document “Working DraftISO/124443”. In the construction shown in FIG. 2 a field coil 7generates a magnetic field having an adjustable field strength. In theabsence of a data carrier this field induces equally high voltages intwo measuring coils 8, 9 which are symmetrically arranged relative tothe field coil 7. The two measuring coils 8, 9 are connected in seriesin phase opposition, so that the difference voltage between the voltagesinduced in the measuring coils is at least substantially zero. Thedifference voltage can be adjusted to a minimum value by means of anadjusting device (not shown).

When a data carrier 10 is introduced into one of the measuring coils,the load in the form of the data carrier 10 causes the voltages inducedin the measuring coils 8, 9 to no longer compensate one another, so thatthe difference voltage indicates the loading by the data carrier 10.Load modulation of the data carrier thus provides amplitude modulationof the difference voltage. Such an amplitude modulated differencevoltage is denoted by the reference UD in FIG. 2.

SUMMARY OF THE DRAWINGS

It has been found in practice that the evaluation of the amplitudemodulation is dependent on the equalization of the measuring coils 8, 9,i.e. that it can be falsified by equalization errors. It is an object ofthe invention to provide a method and a circuit arrangement which enablemore reliable evaluation of a data signal transmitted via loadmodulation.

According to the invention this object is achieved by means of a methodas defined in claim 1 and a circuit arrangement as defined in claim 10.The method according to the invention and the circuit arrangementaccording to the invention utilize the recognition of the fact that theload modulation not only induces modulation of the amplitude, but alsomodulation of the phase. The invention evaluates the amplitudemodulation as well as the phase modulation in common, so that improvedprotection against disturbances can be achieved. Moreover, for measuringpurposes a plurality of transmission parameters can be simultaneouslydetermined in a very simple manner. The detection and determination ofthese parameters can be carried out in automatic measuring devices.

The method according to the invention enables determination of themodulation properties on the basis of the input data signal generated byload modulation. To this end, first a complex data signal is generatedfrom the input data signal. In an advantageous version of the invention,as disclosed in the dependent Claims, such determination can beperformed by quadrature mixing and by assigning the so-called in-phasesignal of a quadrature mixer used to the real part and the quadraturesignal to the imaginary part of a complex envelope. For the complex datasignal, then being present in the form of the complex envelope, adecision line is then determined in the complex plane, i.e. in therepresentation of this signal as a complex locus curve, by making thecomplex envelope approximate a straight line, rotating it through 90° inthe representation of the complex locus curve, and making it extentthrough the mean value of the complex envelope. The decision line thusdivides the complex envelope into two parts for the two load states“loaded” and “unloaded” which can be distinguished in the demodulationof the input data signal generated by load modulation. Finally, the twomean values of the complex envelopes can be determined for these twoload states, and the modulation properties and the input data signal canbe evaluated on the basis thereof.

Instead of evaluating the complex envelope, a complex baseband signalcan also be evaluated. The evaluation can also be performed by digitalor analog Fourier transformation instead of quadrature mixing. It isalso possible to perform sampling in the time domain instead of using aquadrature mixer, i.e. preferably by means of two sample-and-holdcircuits whose sampling instants have been offset by at leastapproximately one quarter of the period duration of the carrieroscillation of the input data signal.

The method according to the invention and the circuit arrangement forcarrying out such a method can thus be very universally used.

In the preferred versions disclosed in the dependent Claims, theamplitude swing resulting from the difference between the absolutevalues of the two mean values for the two states can be determined fromthe mean signal values, separately determined for the loaded state andthe unloaded state, by evaluation of the amplitude modulation. Thisvalue, divided by the sum of the absolute values of said two meanvalues, determines the modulation index.

The phase modulation can be evaluated by determining the mean phaseangles of the input data signal for the two states. The differencebetween these phase angles, i.e. the phase angle between the two complexmean values for the two states, yields the phase swing of the loadmodulation.

Moreover, the absolute value of the difference between the two complexmean values for the loaded state and the unloaded state can be evaluatedas a complex modulation swing.

The invention is implemented preferably in an apparatus for evaluatingan input data signal which is generated by a data carrier by loadmodulation. Such apparatus is formed by measuring equipment; however, itcan also be used in a wide variety of applications in the datacommunication field. In practice a high transmission reliability canthus be achieved for a wide variety of applications.

BRIEF DESCRIPTION OF THE INVENTION

An embodiment of the invention is shown in the FIGS. 3 to 10 of thedrawing and will be described in detail hereinafter. Therein:

FIG. 3 shows a block diagram of a circuit arrangement according to theinvention,

FIG. 4 shows an example of a typical variation in time of the input datasignal,

FIG. 5 shows a complex envelope formed from the input data signal shownin FIG. 4 and subdivided into a real part and an imaginary part,

FIG. 6 shows the complex envelope of FIG. 5 as a complex locus curve,

FIG. 7 shows a diagram illustrating the signal processing steps forevaluating the input data signal or the complex envelope,

FIG. 8 is a more detailed representation of a part of the circuitarrangement shown in FIG. 3,

FIG. 9 is a more detailed representation of a further part of the blockdiagram shown in FIG. 3, and

FIG. 10 shows a further diagram illustrating the signal evaluationaccording to the invention in the form of a complex locus curve.

DETAILED DESCRIPTION

The circuit arrangement shown in FIG. 3 includes an input 11 whichreceives, for example the difference voltage UD of FIG. 2 as its inputdata signal. The difference voltage UD varies in time, for example asshown in FIG. 4. The letter t in this diagram denotes the time. In thetime interval tb the “loaded” state prevails, i.e. the switch 4 in FIG.1 is conductive.

In the time interval tu, however, the “unloaded” state prevails in whichthe switch 4 in FIG. 1 is not conductive. Consequently, in the loadedstate the amplitude of the input signal (difference voltage UD) is lowerthan that in the unloaded state.

The circuit arrangement shown in FIG. 3 also includes a quadrature mixer12 which has a customary construction and includes an in-phase outputand a quadrature output. These two outputs are combined as a complexoutput 13. The signal at the in-phase output of the quadrature mixer 12then describes the real part of the complex envelope of the differencevoltage UD at the input 11; the signal at the quadrature output of thequadrature mixer 12 is assigned to the imaginary part of the complexenvelope. The complex envelope thus formed can be represented as thevariation in time of the real part and of the imaginary part. This isshown in FIG. 5 in which FIG. 5a shows the real part R and FIG. 5b showsthe imaginary part I of the complex envelope as a function of time t.

Another representation of the complex envelope is the complex locuscurve shown in FIG. 6. In the representation as a complex locus curve,the real part R and the imaginary part I define the plane ofrepresentation in which the individual signal values of the complexenvelope are entered according to real part R and imaginary part I. FIG.6 shows a series of signal values for the complex envelope as theyresult, for example, from a signal variation as shown in FIG. 4 and FIG.5, respectively. This representation shows two cumulations of signalvalues which are marked by circles in FIG. 6 and bear the reference Ufor the unloaded state and the reference B for the loaded state.

In order to enable the determination of the modulation properties or toperform automatic separation between the loaded state and the unloadedstate, and also the unambiguous assignment of individual signal valuesrequired for this purpose, the complex envelope in the form of thecomplex locus curve shown in FIG. 6 is subdivided into two parts, onepart being assigned to the unloaded state of the data carrier whereasthe other part is assigned to the loaded state of the data carrier. Suchassignment requires the definition of a decision line which is alsoreferred to as a decision threshold. In order to determine the decisionthreshold, first the mean value of the complex envelope can bedetermined by calculating the mean values separately for the real part Ras well as for the imaginary part I. The resultant mean value M of thecomplex envelope, also representing a complex value, is represented byan arrow in FIG. 6.

The circuit arrangement shown in FIG. 3 includes a mean value detector14 for the described determination of the mean value. The mean valuedetector 14 receives the complex envelope from the complex output 13 ofthe quadrature mixer 12. The mean value detector 14 forms a mean valueRM from the real part R. A mean value IM is independently formed in themean value detector 14 from the imaginary part I of the complexenvelope. The mean values RM and IM together constitute the complex meanvalue M of the complex envelope which is presented on an output 15 ofthe mean value detector 14.

The circuit arrangement shown in FIG. 3 also includes a subtractioncircuit 16, a first input 17 of which receives the complex envelope,separated into the real part R and the imaginary part I, whereas asecond input 18 receives the mean value M of the complex envelope,separated into the real part RM and the imaginary part IM. The output 19of the subtraction circuit 16 thus carries a complex signal without meanvalue, i.e. the complex envelope without mean value. This is shown as alocus curve in FIG. 7, in this case being the representation of theimaginary part I-IM over the real part R-RM of the complex envelopewithout mean value.

The FIGS. 6 and 7 show the decision threshold E as a straight line whichextends between the signal values of the complex envelope for the loadedstate and the unloaded state and through the mean value M of the complexenvelope or through the zero point in the representation of the complexenvelope without mean value. The slope of the decision threshold can bedetermined by making the complex envelope approximate a straight lineand by rotating this straight line through 90° in the locus curverepresentation. The approximating straight line is denoted by thereference G in FIG. 6 and FIG. 7. The slope of the approximatingstraight line G, and hence the decision threshold E, is determinedpreferably by calculation of the least error squares. This isillustrated by FIG. 7. FIG. 8 shows an example of a circuit arrangementfor carrying out this signal operation.

FIG. 8 shows a circuit element which is also referred to as a phasedetector and is denoted by the reference 20 in FIG. 3. The phasedetector 20 is connected to the output 19 of the subtraction circuit 16in order to receive the complex envelope without mean value. A slopesignal is output via an output 21 of the phase detector 20. In order toform this slope signal, the imaginary part I-IM of the complex envelopewithout mean value is applied from the output 19 of the subtractioncircuit 16 to a first input 22 of a first multiplier circuit 23. Thereal part R-RM of the complex envelope without mean value is applied toa second input 24 of the first multiplier circuit 23. The signalcorresponding to the product of these real and imaginary parts isapplied to a first stage 25 for forming a mean value MI of said product.Analogously, the square of the real part R-RM of the complex envelopewithout mean value is formed in a second multiplier circuit 26 so as toform its mean value in a second stage 27. This mean value is referred toas MR. In a subsequent division circuit 28 the quotient of the meanvalues MI and MR is formed. This quotient represents the slope signal onthe output 21. This slope signal provides the value of the slope of thedecision threshold E in the FIGS. 6 and 7. In order to visualize thiscalculation, a first triangle DI in FIG. 7 denotes the slope of theapproximating straight line whereas a second triangle D2 represents theslope of the decision threshold E.

The circuit arrangement shown in FIG. 3 also includes a decision circuit29, also referred to as a decider, which is shown in more detail in FIG.9. In the decider 29 the decision threshold in the complex locus curveof FIG. 6 or FIG. 7 is defined by way of the slope of the decisionthreshold in conformity with the slope signal on the output 21 of thephase detector and the mean value M of the complex envelope. To thisend, the decider 29 includes a second subtraction circuit 30 which,similar to the (first) subtraction circuit 16, first forms the complexenvelope without mean value according to real part and imaginary part bysubtraction of the mean value M from the complex envelope. Its imaginarypart I-IM is applied directly to a first input 31 of a third subtractioncircuit 32. In a third multiplier circuit 33 the real part R-RM of thecomplex envelope without mean value is multiplied by the slope signalfrom the output 21 of the phase detector 20. This product represents thedecision threshold signal whose representation in the complex locuscurve is the straight line E. The decision threshold signal is appliedto a second input 34 of the third subtraction circuit 32 and issubtracted from the imaginary part I-IM of the complex envelope withoutmean value in said third subtraction circuit 32. The result is appliedto a comparison circuit 35 and therein it is checked whether its valueis larger or smaller than zero. The decider thus checks whether theimaginary part of the complex envelope without mean value, also referredto as the imaginary part signal without mean value, lies above or belowthe decision threshold E in the representation of the complex locuscurve, i.e. above or below that value on the decision threshold E whichis determined by the associated value of the real part R-RM of thecomplex envelope without mean value, also referred to as the real partsignal.

The comparison circuit 35 controls a switch 36. The switch 36 connectsthe complex output 13 of the quadrature mixer 12 to a first decideroutput 37 in the case of the state “loaded” whereas the switch 36connects the complex output 13 to a second decider output 38 in the caseof the “unloaded” state.

After the classification of the signal values of the complex envelopeaccording to the loaded state and the unloaded state, a mean valuecalculation can be performed for the two states separately. To this end,the circuit arrangement shown in FIG. 3 also includes a respective meanvalue detector 39 for the loaded state and a mean value detector 40 forthe unloaded state. The corresponding mean values are presented viaoutputs 41 and 42, respectively. The calculation of the mean value inthe mean value detectors 39, 40 can be performed in different ways. Itis notably possible to determine a linear, quadratic or geometrical meanvalue.

In the representation chosen for FIG. 7, for example, a respective meanvalue MB thus determined is entered for the loaded state whereas MU isentered for the unloaded state.

In comparison therewith, FIG. 10 shows the representation of the manvalues MU and MB in a manner similar to FIG. 6, i.e. without subtractionof the mean value M of the complex envelope. The mean values obtained inthis case are denoted as MUP for the unloaded state and as MBP for theloaded state and are represented as pointers in FIG. 10. Thisrepresentation, or a corresponding signal processing, is particularlyattractive for an evaluation of the phase modulation.

The circuit arrangement shown in the FIGS. 3, 8 and 9 can be implementedby for the digital signal processing technique.

What is claimed is:
 1. A method for evaluating an input data signalgenerated by load modulation, characterized in that a complex datasignal which has a first signal component, referred to as a real part,and a second signal component, referred to as an imaginary part, isderived from the input data signal, a mean value signal of the complexdata signal is formed by forming the mean value of the real part on theone hand and of the imaginary part on the other hand, a complex signalwithout mean value, having a real part signal without mean value and animaginary part signal without mean value, is formed by forming thedifference between the complex data signal and the mean value signal ofthe complex data signal, a first quadratic error signal is formed bysignal multiplication of the real part signal without mean value byitself and a second quadratic error signal is formed by signalmultiplication of the real part signal without mean value by theimaginary part signal without mean value, a slope signal is formed bysignal division of the first quadratic error signal by the secondquadratic error signal, a decision threshold signal is formed by signalmultiplication of the slope signal by the real part signal without meanvalue, and an information signal as to whether a value of the input datasignal has been formed in a loaded state or an unloaded state during theload modulation is formed by comparing the imaginary part signal withoutmean value with the decision threshold signal.
 2. A method as claimed inclaim 1, characterized in that an in-phase signal and a quadraturesignal are formed by quadrature mixing in order to form the complex datasignal, the in-phase signal constituting the real part and thequadrature signal constituting the imaginary part of the data signalwhich is referred to as a complex envelope.
 3. A method as claimed inclaim 1, characterized in that from the input data signal a complexbaseband signal is derived as the complex data signal.
 4. A method asclaimed in claim 1, characterized in that an in-phase signal and aquadrature signal are formed by Fourier transformation in order to formthe complex data signal, the in-phase signal constituting the real partand the quadrature signal constituting the imaginary part of the datasignal which is referred to as a complex envelope.
 5. A method asclaimed in claim 1, characterized in that the real part and theimaginary part are derived by sampling the input data signal at twoinstants which are offset relative to one another by at leastapproximately one quarter period of a carrier oscillation of the inputdata signal.
 6. A method as claimed in claim 1, characterized in thatsignal values of the complex data signal are determined separately forthe loaded state and the unloaded state.
 7. A method as claimed in claim6, characterized in that an amplitude swing signal and/or a modulationindex signal is derived from the signal values separately determined forthe loaded state and the unloaded state.
 8. A method as claimed in claim6, characterized in that a phase angle signal and/or a phase swingsignal is derived from the signal values separately determined for theloaded state and the unloaded state.
 9. A method as claimed in claim 6,characterized in that a complex modulation swing signal is derived fromthe signal values separately determined for the loaded state and theunloaded state.
 10. A circuit arrangement for evaluating an input datasignal generated by a load modulation, the circuit comprising: a datasignal generating circuit for forming from the input data signal acomplex data signal which has a first signal component, referred to as areal part, and a second signal component, referred to as an imaginarypart; a mean value detector for forming a mean value signal of thecomplex data signal by forming a first mean value of the real part onthe one hand and a second mean value of the imaginary part on the otherhand; a subtraction circuit for forming a complex signal without meanvalue, having a real part signal without mean value and an imaginarypart signal without mean value by forming a difference between thecomplex data signal and the mean value signal of the complex datasignal; a multiplier circuit for forming a first quadratic error signalby signal multiplication of the real part signal without mean value byitself, for forming a second quadratic error signal by signalmultiplication of the real part signal without mean value by theimaginary part signal without mean value, and for forming a decisionthreshold signal by signal multiplication of a slope signal by the realpart signal without mean value; a division circuit for forming the slopesignal by signal division of the first quadratic error signal by thesecond quadratic error signal; a comparison circuit for forming aninformation signal by comparing the imaginary part signal without meanwith the decision threshold signal, the information signal indicatingwhether a value of the input data signal has been formed in a loadedstate or an unloaded state.
 11. A circuit arrangement as claimed inclaim 10, wherein the data signal generating circuit includes aquadrature mixer.
 12. A circuit arrangement as claimed in claim 10,wherein the data signal generating circuit includes a Fouriertransformation circuit.
 13. A circuit arrangement as claimed in claim10, wherein the data signal generating circuit includes twosample-and-hold circuits which are controlled by means of a clock signalderived from a frequency of a carrier oscillation of the input datasignal.
 14. A circuit arrangement as claimed in claim 10, furthercomprising: a further mean value detector for the separate determinationof the mean value of signal values of the complex data signal for theloaded state and the unloaded state.
 15. A circuit arrangement asclaimed in claim 14, further comprising: a signal evaluation circuit fordetermining the amplitude modulation and/or the phase modulation and/orthe modulation swing.
 16. An apparatus for evaluating an input datasignal generated by a data carrier by load modulation, the apparatuscomprising: a data signal generating circuit for forming from the inputdata signal a complex data, signal which has a first signal component,referred to as a real part, and a second signal component, referred toas an imaginary part; a mean value detector for forming a mean valuesignal of the complex data signal by forming a first mean value of thereal part on the one hand and a second mean value of the imaginary parton the other hand; a subtraction circuit for forming a complex signalwithout mean value, having a real part signal without mean value and animaginary part signal without mean value by forming a difference betweenthe complex data signal and the mean value signal of the complex datasignal; a multiplier circuit for forming a first quadratic error signalby signal multiplication of the real part signal without mean value byitself, for forming a second quadratic error signal by signalmultiplication of the real part signal without mean value by theimaginary part signal without mean value, and for forming a decisionthreshold signal by signal multiplication of a slope signal by the realpart signal without mean value; a division circuit for forming the slopesignal by signal division of the first quadratic error signal by thesecond quadratic error signal; a comparison circuit for forming aninformation signal by comparing the imaginary part signal without meanwith the decision threshold signal, the information signal indicatingwhether a value of the input data signal has been formed in a loadedstate or an unloaded state.